Energy storage is a crucial component to increasingly large number situations, e.g., in powering electrical vehicles, portable devices, and storage and mobile use of solar power. The current state of the art has electrical energy stored, e.g., in a battery in chemical form, while in a capacitor it is in an electric field and polarization of a dielectric material, it is in magnetic form in a superconducting loop, in mechanical form in a flywheel generator combination, or gravitational form, in a hydroelectric installation.
The current bench marks for energy sources have nuclear at 1.5×1018 J/m3 (Uranium breeder reactor) while for fossil fuels ≈4×1010 J/m3 (Kerosene) and current Li-ion chemical batteries ≈1×109 J/m3 and ≈5×107 J/m3 for super-capacitors. Excluding nuclear power and given fossil fuel engines are less than 50% efficient, an idea electrical battery would correspond to an energy density ≈2×1010 J/m3 and a high-power density, i.e., large currents upon charge and discharge.
Apart from nuclear, all these sources of energy correspond to atomic physics and this implies certain general limits on the energy density. For example, the discharge of a chemical battery involves the transfer of electrons between the outer orbitals of atoms. The energies involved are scaled by the Rydberg 13.6 eV, that is to say that the transfer of an electron involves an energy that are invariably less than or of the order of this and corresponding to a difference of electrical potential difference of about 13.6V.
Since the open circuit terminal voltage is limited by this potential difference, a battery cannot not have a terminal voltage of much more than 10V and cannot store more than about 10 eV per atom. A typical energy storage material, e.g., LiCoO2 has about 1028 atoms/m3 and the ideal limit is 10V×1.6×10−19 J/V×1028→1.6×1010 J/m3. If the electronic process involves 2 or 3 electrons per atom, there is a potential gain by the corresponding multiple. The 1.6×1010 J/m3 is about a factor of twenty greater than the best chemical Li-ion battery today. Since the electric field that a material can withstand without breakdown is similarly limited, this estimate also applies to capacitors and the phase transition battery described here. If energy is measured in, e.g., kWhr/kg then lighter elements such as Li, Na, Mg or Al are favored by factors of 2-3 over the 3d transition metals such as Co or Fe.
Chemical batteries have a limited power density that reflects the slow ionic transport and results in poor charge and discharge performance. Furthermore, the reliance of existing batteries on ionic transport invariably causes high degradation rates of the batteries. On the other hand, capacitors involve rapid electronic transport with a correspondingly higher power density but have a very small fraction of useful working material and correspondingly small energy densities.
What is needed is an improved energy storage technology.